Ophthalmoscopy (funduscopy or fundoscopy) is a test that allows a clinician to see inside the fundus of the eye and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination and is crucial in determining the health of the retina and the vitreous humor. A Direct Ophthalmoscope produces an upright, or unreversed, image of approximately 15 times magnification. The indirect ophthalmoscope produces an inverted, or reversed, direct image of 2 to 5 times magnification and allows for wider view of the inside of the eye that is useful in detecting several eye related diseases like glaucoma, diabetic retinopathy or eye related imperfections like retinal detachment etc. A diagrammatic representation of the eye is shown in FIG. 1 with the key features of the eye 10 as pupil 12, cornea 14, iris 16, anterior chamber 18, posterior chamber 20, eye lens 22, retina 24, and vitreous humour 26, fovea 28 and optic disc 30. The fundus 32 of the eye is the interior surface of the eye, opposite the lens, and includes the retina, optic disc, macula and fovea, and a few other features.
One of the important aspects in the development of retinal imaging devices relates to solving the issue of the reflections from cornea and the illumination lens. To solve this issue, the Gullstrand Principle for reflex-free ophthalmoscope has been employed in several versions of the eye imaging devices. The principle broadly involves separating the entrance and exit of light in the plane of the pupil of the eye. Therefore the paths of illumination and imaging need to be separated from the cornea to the pupil. More specifically the light entering the eye and imaging light coming out of the eye at the anterior segment (between front of cornea till back of lens) needs to have separate paths. Some exemplary developments in this field are listed below.
U.S. Pat. No. 3,594,071 describes an ophthalmoscopic camera which can eliminate undesired light reflected from the cornea of an eye to be examined and the front and back surfaces of a front objective through which the illumination light passes by interposing a ring-shaped aperture between a plane reflecting mirror and a condenser lens arranged next to the reflecting mirror and a small shield between said condenser lens and the next condenser lens so as to shield a small area in the vicinity of and including the optical axis. The optical system for photography includes no such shield as described above and said photographic objective is of biconvex, thereby increasing the picture angle to 45.degree
U.S. Pat. No. 5,713,047 provides an eye fundus photographing apparatus that has an illumination system for illuminating an eye fundus under test by an illumination light beam, a photographing system for photographing the eye fundus under test by a photographing light beam from the eye fundus under test illuminated by the illumination system, and a light shielding member arranged at a vicinity of the illumination diaphragm or the photographing diaphragm or at a vicinity of a conjugate position of the diaphragms, such that the eye fundus photographing apparatus enables eye fundus photographing to be in a good condition to an eye under test having a small pupil diameter.
U.S. 5,572,266 describes a fundus camera capable of adjusting a working distance between a camera body and a subject's eye quickly and easily with high accuracy and capable of observing an eye fundus image in a field of view and at a magnification each substantially same as those selected when an eye fundus is photographed. To adjust the working distance quickly and easily with high accuracy, the fundus camera has an optical member provided in a projecting optical system for projecting alignment light onto a subject's eye. The optical member projects split alignment images onto the eye when the working distance is out of a predetermined proper distance. To observe the eye fundus image in the substantially same field of view and at the substantially same magnification as those selected when the eye fundus is photographed, the fundus camera has a device for, according to power variation, changing a state of illumination light illuminating the eye. In the fundus camera, the alignment light is projected onto the eye, the alignment light reflected by the eye is once converged on a point of a photographic optical path conjugate with the fundus of the eye, the reflected alignment light converged thereon is guided to a TV monitor through a variable power lens, and the state of the illumination light illuminating the eye is changed by the changing device to observe and photograph the eye fundus.
U.S. Pat. No. 4,838,680 describes an optical system for a retinal camera that comprises objective lens adapted to be located toward a patient's eye. The retina of the patient's eye is illuminated and photographed through the objective lens, which has a first biconvex type lens means and a second biconvex type lens means with a space between them, the first biconvex type lens means has a curvature radius of the first convex surface open to the air and faces toward the eye of which is larger than that of the other second convex surfaces open to the air, and the second biconvex type lens means of which curvature radius of third convex surface open to the air and facing toward the first biconvex type lens means is larger than that of the other fourth convex surfaces open to the air.
U.S. Pat. No. 6,546,198 describes a fundus camera that includes a main unit equipped with an illuminating optical system for illuminating the fundus of an eye to be inspected and a photographing optical system for photographing the fundus, an observation optical system for determining whether or not the distance between the main unit and the eye is equal to a proper working distance, an alignment index projecting system, and a driving mechanism for moving a light guide along an optical axis to change a working distance in the case of photographing the central part of the fundus and in the case of photographing a peripheral part of the fundus.
U.S. Pat. No. 7,275,826 describes an apparatus for obtaining an image of the eye, whereon the apparatus comprises a light source for providing an incident illumination and an apertured mirror for directing at least a portion of the incident illumination along an optical axis. A curved objective mirror directs the incident illumination received along the optical axis toward the retina of the eye and directs image-bearing light reflected from the retina back along the optical axis. The apertured mirror transmits the image-bearing light reflected from the retina toward a sensor for obtaining an image of the retina thereby.
U.S. Pat. No. 6,296,358 provides an ocular fundus imager that automatically aligns fundus illuminating rays to enter the pupil and to prevent corneal reflections from obscuring the fundus image produced. Focusing the produced fundus image is automatically performed and is based upon the fundus image itself. A head restraint for the patient undergoing examination is in the form of a pair of spectacles which is not only easy to use accurately but significantly reduces the gross alignment between the optical system and the patient's pupil.
U.S. Pat. No. 4,874,236(A) describes another ophthalmologic apparatus for fundus examination comprising a base, an illumination device and a microscope carried by the base and rotatable about a vertical axis, a fixture for the patient's head and eyes with respect to the vertical axis and a lens positioned at the upper end of the vertical rod in the optical axis of the microscope and the examined eye and displaced from the vertical axis towards the examined eye such as to image the retina of the examined eye at a location for reimaging it by the microscope.
U.S. Pat. No. 4,235,529 describes an apparatus for taking photographs of crystalline lens sections including a slit illumination system and a photographing optical system which includes an optical axis inclined with respect to the slit illumination plane. In order to eliminate an image of illumination light source which may be produced by a light reflected at the patient's cornea, a light interrupting blade member is provided in the illumination optical path at a side of the slit axis adjacent to the photographing optical system.
Though fundus imaging devices have evolved and there have been several improvements in devices for imaging the eye, most of them require high level of operator skill and complex instrumentation. One significant issue that continues to be a problem is the need to eliminate artifactual reflections from the patient's eye as well as from the lens surfaces used in the optical design of the illuminating and imaging apparatus itself, using as simple and compact an optical design as possible. Unless the level of these artifactual reflections is reduced significantly, the overall image quality and image contrast is affected. Current prior art, for example the use of crossed polarizers (see U.S. Pat. No. 7,275,826 FIG. 1 for prior art discussion), eliminates the background reflections to some extent but also significantly reduce the reflectance signal from the retina as well. The use of one or more ring shields in front of the illuminating source (see U.S. Pat. No. 3,594,071 FIG. 1), help reduce the reflections, but increase the size of the optical illumination path. The use of simple perforated mirrors have helped separate the imaging and illumination paths, but can in some design embodiments result in both surface reflections as well as stray illumination reflections from still reaching the imaging sensor/imaging eyepiece. Thus there continues to be a need for further improvement that can help reduce unwanted reflections to improve image quality while keeping the overall optical design of the system, simple and compact.